1. Field:
This invention relates to a method for treating coal, and more particularly to a method for separating resin from coal by froth flotation.
2. State of the Art:
Using froth flotation to remove and separate natural resin ("resins") from resin-bearing coal is well known. U.S. Pat. No. 1,773,997 (Green) is illustrative of the process. Resin-bearing coal is comminuted in the presence of two parts of water and passed through a 40 mesh screen. Another part of water is added, and the resulting pulp is introduced into a froth flotation machine. A frothing agent ("frother") such as amyl alcohol, is added to the pulp, and the mixture agitated. A froth forms which carries with it a certain portion of resin while the coal-predominant tailing remains.
The resultant resin-bearing froth is then retreated. The resinous froth is reintroduced into the flotation machine, water is added to fill the machine to a working level, and substances such as potassium alum, may be added. The resulting mixture is then agitated. More frothing agent is introduced, and a resin froth is taken from the flotation machine. Green reported that relatively pure (approaching 96% by weight) resin can be attained using the above two-step method.
Extracting resin from resin-bearing coal is particularly important in Western North America. There, certain coal fields contain significant quantities of resins. Other areas of the world having resin-bearing coal include Mainland China and Argentina.
The State of Utah has significant amounts of resin-bearing coal in its coal fields. The resins obtained from Utah coal generally have low specific gravities, approximately 1.03 g/cm.sup.3, and vary in color from lemon yellow to almost black. The resins are important commercially, being used for adhesives, varnishes, coatings, waterproofing, linoleum, etc., and command a substantially higher price per pound as a chemical commodity than as a fuel.
Most prior art flotation techniques for resin recovery have not been particularly selective. Both components of the feed, resin and coal, have similar hydrophobic characteristics as shown by the contact angle and bubble attachment time data listed in Table 1. The difference between the bubble attachment times for resin and coal is insufficient to achieve the desired selective flotation separation by conventional flotation techniques.
The hydrophobic character of resin and coal from the Hiawatha, Utah seam is shown in Table 1. The bubble attachment time was measured at a particle bed with particle size 212.times.300 microns at a pH of 6.5.
TABLE 1 ______________________________________ Contact Angle Bubble Attachment Component Degrees Time (ms) ______________________________________ Resin 58-59 5 Coal 48-51 15 ______________________________________
U.S. Pat. No. 4,724,071 to Miller et al. discloses a selective flotation process in which coal particles can be selectively depressed by ozone conditioning. In this flotation process, a resin concentrate product which contains 95% resin at a recovery of 70-80% can be obtained in single stage flotation from resinous coal feed containing approximately 10% resin. Such a high purity concentrate product can be directly used as feed stock for some industrial applications cited before and the costly solvent refining processcan be eliminated in many cases. However, this ozone flotation process uses resinous coal ground to at least minus 200 mesh prior to ozone conditioning. Generally such grinding is not practiced at coal preparation plants.